Field
The present application relates generally to obtaining tomographic images in a dental environment, and, more particularly, to methods, systems, apparatuses, and computer programs for removing artifacts from a tomosynthesis dataset.
Description of Related Art
X-ray radiography can be performed by positioning an x-ray source on one side of an object (e.g., a patient or a portion thereof) and causing the x-ray source to emit x-rays through the object and toward an x-ray detector (e.g., radiographic film, an electronic digital detector, or a photostimulable phosphor plate) located on the other side of the object. As the x-rays pass through the object from the x-ray source, the x-rays are transmitted to varying degrees depending on the composition of the object and the energy of the x-rays, and x-rays arriving at the x-ray detector form a two-dimensional (2D) x-ray image (also known as a radiograph) based on the cumulative x-ray attenuation through the object. Thus, a single radiograph does not provide sufficient depth information about features within an object. Features often appear to overlap in a conventional radiograph, although in the object in three-dimensional (3D) space, the features are separate.
X-ray radiography can be performed in dentistry, and dental x-ray radiography systems typically include an x-ray source suspended from a wall-mounted arm and an intraoral x-ray sensor. Dental x-ray radiography systems are relatively compact and conveniently can be used chair-side to provide guidance during a treatment, such as an endodontic procedure. However, dental x-ray radiography also does not provide depth information concerning a patient's unique internal anatomical features (e.g., the shape of dental root structures), although such depth information often would be useful in diagnosing and treating dental pathologies.
X-ray computed tomography (CT), and more particularly cone beam computed tomography (CBCT), has been used to acquire 3D data about a patient, which includes depth information. Dental CBCT is performed by rotating an x-ray source and an x-ray detector with a large field of view (typically, at least large enough to image a patient's complete jaw) through a scan angle of at least 180° around a patient's head, while the patient is sitting or standing in the machine. The 3D data acquired by CBCT can be presented on a display screen for clinician review as a 3D rendering or as a stack of parallel 2D tomographic image slices, each slice representing a cross-section of the patient's anatomy at a specified depth. However, CBCT machines carry a high cost of ownership, are too large for use in chair-side imaging and expose patients to a relatively high dose of x-rays (due to the large field of view, the at least 180° scan angle, and the penetration of x-rays through the complete jaw).
Tomosynthesis is an emerging imaging modality that provides 3D information about a patient in the form of 2D tomographic image slices reconstructed from projection images taken of the patient with an x-ray source from multiple perspectives within a scan angle smaller than that of CBCT (e.g., ±20°, compared with at least 180° in CBCT). Tomosynthesis systems are commercially available for mammographic imaging. Tomosynthesis as an imaging modality can also be applied to intraoral imaging.
Tomosynthesis image slices are reconstructed by processing the projection images taken in a model of the geometry of the tomosynthesis system (e.g., the relative position and angle of the imaged object in 3D space). Spatial instability in the geometry of the tomosynthesis system and/or the object can result in misaligned projection images that deviate from the aforementioned model, which, in turn, can degrade the quality and spatial resolution of the reconstructed tomosynthesis image slices. For example, spatial instability in intraoral tomosynthesis imaging can arise from motion, whether intentional or unintentional, of the patient, the arm-mounted source, and/or the intraoral detector.
Projection images affected by motion or other spatial instabilities can be aligned prior to reconstruction so as to compensate for such motion or instability. One solution is adapted from CBCT imaging to tomosynthesis imaging, namely, placing radiopaque markers within the field of view of the tomosynthesis imaging system, which create visible guides in the projection images that can facilitate realignment of the projection images. For example, three or more highly attenuating marker particles of a known geometry can be placed within the field of view of the imaging system to allow for simple computation of a sample position for a fixed x-ray source and a fixed detector geometry. However, because the field of view in intraoral tomosynthesis is typically smaller than that of CBCT, it can be difficult to place the radiopaque markers in a location that does not obscure relevant anatomical detail in the projection images. In particular, an intraoral sensor is generally restricted to a usable projection angle of ±45 degrees. Thus, objects are typically significantly blurred along the depth directions. Moreover, because intraoral sensors must be small enough to fit inside a subject's mouth, the radiopaque markers tend to obscure relevant dental anatomy wherever the markers are placed.
Additionally, the high contrast edges of radiopaque markers in the projection images can result in major reconstruction artifacts (e.g., out-of-plane contributions and in-plane ringing) in the tomosynthesis images. For example, in back-projection methods, the use of highly attenuating marker particles can result in image artifacts, which are typically characterized by strong streaking throughout the imaged volume due to large image gradients being back-projected through the volume, as well as dimming of imaged features that are obscured by the marker particles. However, in full-angle tomosynthesis imaging, image artifacts are generally benign because any artifacts generated by the marker particles will typically only obscure a small fraction of the imaged volume.
Another solution is to align the projection images based on inherent details and features of the projection images, which is also known as markerless alignment, in contrast to the above-mentioned technique of using radiopaque markers as artificial alignment landmarks. However, the accuracy of markerless alignment is typically limited by the extent in depth of the image features used for alignment. Certain types of imaged anatomy, such as teeth and trabeculae, may contain large, irregular features that are not conducive to markerless alignment.